The present invention relates to a resin-molded prism with a good optical performance and manufactured by injection molding capable of high-precision positioning, and a mold for injection-molding this resin-molded prism.
A method of manufacturing an optical component in accordance with injection molding by using a resin has been developed aiming at reduction in cost and weight of the optical component. When an optical component is to be manufactured by injection molding, elements for attaching and positioning with respect to an optical device can be molded integrally with the optical component, so the optical component can be assembled and positioned easily, and the number of components can be reduced. For example, Japanese Patent No. 2778651 discloses an arrangement in which a positioning portion for positioning an optical prism to be manufactured by injection molding is formed integrally with the optical prism.
In injection molding of an optical component, it should be noted that when a resin in the molten state passes through the gate and fills a cavity formed in a mold, no flow marks should be formed by inclusion of the resin. Based on this viewpoint, according to the technique disclosed in Japanese Patent Laid-Open No. 10-305458, when injection-molding a block-like optical component such as a prism, a gate is connected to the ridgeline of two adjacent surfaces of a molded product, and the direction of the gate is inclined equally with respect to the two surfaces, so flow marks which are formed when a molten resin is to fill a cavity are suppressed.
FIGS. 6 to 8 schematically show injection molding procedures disclosed in Japanese Patent Laid-Open No. 10-305458. More specifically, when a gate 4 is connected to the ridgeline of two adjacent molding surfaces 2 and 3 of a cavity 1 and the extending direction of the gate 4 is inclined equally with respect to the two molding surfaces 2 and 3, as the sectional area of the resin flow path from the gate 4 to the cavity 1 rapidly increases, resin separation A (see FIG. 7) from the molding surfaces 2 and 3 and flow disorder occur during filling of a molten resin 5 into the cavity 1. Then, when filling is ended, flow marks 6 (see FIG. 8) and a weld sometimes occur in the optical component. Regarding these drawbacks, if the filling rate of the resin with respect to the cavity 1 is decreased, a region which is located in the vicinity of the gate 4 to adversely affect the optical performance of the optical component can be decreased, but it is difficult to completely eliminate this region. When the filling rate decreases, the transfer performance decreases, and a high-precision optical surface cannot be molded. Also, it can be easily anticipated that a shear strain during filling of the resin is large in the vicinity of the gate 4, so the resin solidifies directly likely to cause residual stress.
In a thick molded product such as a prism, when the cavity 1 is filled with the molten resin 5, a dwell process of replenishing the molten resin 5 from the gate 4 with an appropriate pressure is generally performed even after filling is completed, so a sink mark is prevented from being formed by resin shrinkage that accompanies cooling. Since resin replenishment in the dwell process is performed while cooling of the gate 4 and its vicinity progresses and the resin viscosity increases, a residual stress tends to occur due to the shear strain even during the dwell process. The residual stress caused in this manner not only causes birefringence to degrade the optical characteristics of the optical component but also causes a change in the shape of the optical surface by stress relaxation when the optical component is in use for a long period of time.
It is an object of the present invention to provide a resin-molded prism with two or more optical surfaces, in which an adverse influence of a flow mark or residual stress occurring in the vicinity of a gate is eliminated so as to provide good optical characteristics, and moldability of which can be improved by moderating injection molding conditions, and a mold for injection-molding this resin-molded prism.
The first aspect of the present invention resides in a resin-molded prism characterized by comprising not less than two optical surfaces that are not parallel to each other, a pair of side end faces substantially perpendicular to the optical surfaces, at least one reference block projecting from either one of the pair of side end faces and having a circumferential end face extending in a projecting direction of the reference block, a positioning portion formed on the circumferential end face of the reference block, and a curved surface for smoothly connecting the circumferential end face of the reference block and corresponding one of the pair of side end faces, wherein the circumferential end face and the corresponding one of the side end faces form an angle of not less than 90xc2x0.
According to the present invention, a molten resin fills a cavity formed in a mold that molds a prism itself, through a cavity formed in a mold that molds a reference block. In this case, the molten resin flowing in the longitudinal direction of the circumferential end face of the reference block does not easily separate from the inner wall of the mold in the presence of the curved surface which smoothly connects this circumferential end face and the side end face to each other. Even if resin separation or disorder of flow occurs at the outlet of the gate, its influence is limited within the cavity that forms the reference block, and does not act on the cavity that molds the prism itself.
The second aspect of the present invention resides in a mold characterized by having a cavity conforming to an outline of the resin-molded prism according to the first aspect, the mold comprising not less than two optical surfaces that are not parallel to each other, a pair of side end faces substantially perpendicular to the optical surfaces, at least one reference block projecting from either one of the pair of side end faces and having a circumferential end face extending in a projecting direction of the reference block, a positioning portion formed on the circumferential end face of the reference block, and a cavity surrounded by a molding surface that molds a curved surface for smoothly connecting the circumferential end face of the reference block and corresponding one of the pair of side end faces, wherein the circumferential end face and the corresponding one of the side end faces form an angle of not less than 90xc2x0.
Other objects and advantages beside those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.